1. Technical Field
The present invention relates to a four-wheel drive vehicle, and more specifically, a four-wheel drive vehicle that is able to change both the distribution of driving force to front and rear wheels and the differential-limiting control of left and right drive wheels according to the running state of the vehicle.
2. Background Art
In some types of four-wheel drive vehicles, engine driving force is preferentially applied to rear wheels as main drive wheels in a normal condition such that the vehicle is driven only by the rear wheels, and in the case of slippage of these rear wheels, the engine driving force is also distributed to front wheels as slaved drive wheels such that the vehicle is driven by four wheels (what is called a four-wheel drive vehicle of FR base). In these types of four-wheel drive vehicles, output of a transmission is transferred to (first) differential mechanism (transfer mechanism) with differential-limiting functions and the most desirable distribution of torque adapted to the running state can be achieved by controlling differential-limiting by this first differential mechanism. A hydraulic multiple disc clutch is known as means for this differential-limiting, as shown in Japanese Patent Application 63-141831.
The driving force distributed to the main drive wheels (the rear wheels) is further distributed to each of the left and right wheels by way of another (second) differential mechanism. It is possible to construct this second differential mechanism as a limited slip differential by adding differential-limiting means such that the drive force of the vehicle can be reliably maintained even when slippage or the like occurs in one of the rear wheels. A hydraulic multiple disc clutch of a type mentioned above may be used as this differential-limiting means, as shown in Japanese Patent Application 62-103227. By controlling clutching force of the clutch and adjusting its differential-limiting force, a considerably desirable differential control adapted to the running state of the vehicle can be achieved.
Generally, the first differential mechanism described above controls differential limiting according to the difference between average rotational speed of the front wheels and that of the rear wheels, while the second differential mechanism controls differential-limiting according to the difference between rotational speed of the left rear wheel and that of the right rear wheel.
But when the first differential mechanism and the second differential mechanism are used in combination, problems as below may arise. Upon the occurrence of rotational speed difference between the left and right rear wheels (due to a turn or the like), the second differential mechanism performs differential-limiting control such that the rotational speed difference between the two rear wheels is eliminated, and after the elimination of the speed difference the differential-limiting force is removed. However, if the state of road and running (or the state of road and running that causes a rotational speed difference between the rear wheels) does not change, a rotational speed difference will probably occur again making the second differential mechanism operate another differential-limiting control, and the repetition of this will cause what is called hunting. This hunting problem may also occur when differential-limiting control by the second differential mechanism cannot catch up the changes in the road/running state. Once hunting occurs, it may change the rotational speed difference between the front wheels and the rear wheels, resulting in another hunting in the differential-limiting control by the first differential mechanism. As a result, the vehicle may experience irregular states such as oversteer or understeer and operational stability will significantly deteriorate.